The invention is based on a helical compression spring for use in a fuel injection system.
DE 195 47 424 A1 has disclosed a helical compression spring in which wire with a circular cross section is wound to produce a helical compression spring, which is then flattened at its ends. The helical compression spring is disposed in a component of a fuel injection system and acts on a control part, for example a valve member in a fuel injection valve. A valve member of this kind has a pressure face, which is acted on by the highly pressurized fuel, and by means of the hydraulic force thus produced, can be moved counter to the force of the helical compression spring, as a result of which the valve member controls the injection of fuel into the combustion chamber of an internal combustion engine. Since the fuel pressure in fuel injection valves of the kind used to inject fuel into the combustion chamber of an auto-ignition engine is very high-up to 200 MPa, powerful forces act on the valve member so that the helical compression spring must exert a correspondingly powerful opposing force. On the other hand, since the fuel injection valve, like all other components of the fuel injection system, should be compact in design, a helical compression spring is needed, which has a low winding ratio.
The round wire used previously has the disadvantage that the shearing strains in the inner region of the spring wire of the helical compression spring become relatively intense when loaded with pressure, which makes it impossible to reduce the diameter of the helical compression spring beyond a certain value. DE 195 47 102 A1 has disclosed a helical compression spring that is in fact produced from a round wire, but after being wound, the helical compression spring is ground somewhat on the outside. This achieves virtually the same spring constant with a smaller diameter, since the outer region of the helical compression spring is not subject to any intense stresses and consequently makes hardly any contribution to the total rigidity of the helical compression spring, but there is still the disadvantage that the stresses on the inside of the helical compression spring remain great. Moreover, like all round wire springs, this one has the disadvantage that the initial stress of the helical compression spring decreases with time and the opening pressure of the valve member decreases along with it. This occurs due to the fact that a flat contact surface has to be provided, which is aligned perpendicular to the longitudinal axis of the helical compression spring. The last two loops of the helical compression spring contact each other over a part of their length so that the spring wire of the penultimate loop has a linear contact with the spring wire of the last loop. This pressure produces locally intense mechanical stresses, which in connection with relative movements, can produce vibrational wear on the helical compression spring at this location. This leads to a flattening of the spring wire there until a flat contact of the relevant spring loops is produced. As a result, the helical compression spring shortens somewhat and the opening pressure of the fuel injection valve decreases due to the decreasing initial stress of the helical compression spring.
The prior art has also disclosed helical compression springs that have a rectangular spring wire cross section, for example in DE 37 01 016 A. A helical compression spring of this kind does in fact solve the problem of the high pressure and the resulting decrease in opening pressure of fuel injection valves, but the mechanical stress distribution on the internal diameter of the spring is unfavorable. In order to avoid powerful stresses, attention must also be paid here that the winding ratio is not selected as too low.
The helical compression spring according to the invention has the advantage over the prior art that due to the optimized spring wire cross section, a compressive initial stress of the helical compression spring is not reduced or is only reduced to an insignificant degree by wear between the wire ends and the respective loops adjoining them and that with the same external dimensions, the helical compression spring produces greater spring forces than a helical compression spring with a circular spring wire cross section. The sides of the spring wire, which are oriented toward each other, are surfaces that are embodied as at least approximately parallel to each other so that at the ends of the helical compression spring, between the wire ends and the adjacent loops of the spring wire, a flat contact is achieved, which reduces wear and therefore sharply reduces a decrease in the opening pressure of the fuel injection valve.
In one advantageous embodiment, the spring wire of the helical compression spring has a cross section, which starts out square and is then rounded, and whose side constituting the inside of the helical compression spring is convexly curved. When the helical compression spring is compressed and also as a result of the initial stress, chiefly shearing stresses occur in the helical compression spring, which reach their maximal intensity on the inside of the helical compression spring. Due to the powerful forces to which the helical compression spring is subjected, for example in a fuel injection valve, powerful shearing stresses occur in the spring wire, which must not exceed certain maximal values. As a result, with a predetermined opening pressure and opening stroke of the valve member of a fuel injection valve, the helical compression spring cannot exceed a certain length. Because of the changed cross section, the helical compression spring according to the invention reduces the maximal shearing stress so that greater spring forces can be achieved with the same length. Alternatively, this circumstance can also be used to produce a shorter helical compression spring without changing the spring forces or the spring constant so that the fuel injection valve can be designed to be correspondingly shorter.
In order to obtain a spring with the cross section according to the invention, a wire must be used, which has a cross section that differs from it, since the cross section of the spring wire changes when being wound into the helical compression spring. With the spring wire according to the invention, the side surfaces that are oriented toward each other after the winding of the helical compression spring are embodied as inclined in relation to each other. This inclination of the side surfaces advantageously results in the fact that the side surfaces are aligned at least approximately parallel to each other when the helical compression spring is wound and then have the above-described advantages without requiring an expensive, complex secondary processing of the helical compression spring after winding.
The helical compression spring according to the invention can also be provided for use in another component of a fuel injection system. For example, there are also control parts in high-pressure fuel pumps, which are hydraulically moved by the fuel pressure counter to the force of a helical compression spring. Since it is important herexe2x80x94as in all components of the fuel injection systemxe2x80x94to use the most compact, space-saving design possible, advantageous use can be made here of the helical compression spring according to the invention.
Other advantages and advantageous embodiments of the subject of the invention can be inferred from the drawings, the specification, and the claims.